---
id: PLN-0033
ticket: perf-vm-allocation-and-copy-pressure
title: Plan - VM Hot Path Ownership and String Copy Pressure
status: done
created: 2026-04-20
completed: 2026-04-20
tags: [perf, runtime, vm, memory, strings]
---

## Briefing

Implement the runtime-core portion of `DEC-0018` by reducing repeated string payload copies and hot-path allocation pressure without changing guest-visible semantics.

## Decisions de Origem

- `DEC-0018` - VM Allocation and Copy Pressure Baseline

## Alvo

Change the VM core so that hot paths over numeric values and already-materialized values converge toward zero allocation, while preserving the public semantics of string values and `GET_GLOBAL`.

## Escopo

### Included

- VM value representation and ownership audit for hot paths.
- `GET_GLOBAL` copy-pressure reduction without semantic drift.
- String-producing opcode hot-path changes, especially `ADD`.
- Unit and focused integration coverage for unchanged semantics.

### Excluded

- Public ABI changes.
- Certification or host-facing telemetry policy changes.
- Broad speculative string interning or copy-on-write rollout unless directly required by a concrete implementation step.

## Fora de Escopo

- Cartridge language changes.
- Host debugger protocol changes.
- General fault/log path rewrites beyond keeping them isolated from hot-path work.

## Plano de Execucao

### Step 1 - Audit current hot-path ownership

**What:** Identify the exact VM operations that still clone or allocate on already-materialized hot paths.
**How:** Review `Value`, `GET_GLOBAL`, `ADD`, related stack helpers, and any nearby string coercion paths; classify each site as unavoidable first materialization versus avoidable repeated copy.
**File(s):** `crates/console/prometeu-bytecode/src/value.rs`, `crates/console/prometeu-vm/src/virtual_machine.rs`

### Step 2 - Rework internal representation/ownership for expensive payloads

**What:** Introduce the smallest internal representation change needed to avoid repeated payload copies for expensive values.
**How:** Prefer internal ownership changes that preserve guest-visible semantics. If strings or other expensive payloads need to move behind handles or equivalent internal storage, keep the public value meaning unchanged and constrain the change to runtime-core internals.
**File(s):** `crates/console/prometeu-bytecode/src/value.rs`, `crates/console/prometeu-vm/src/heap.rs`, `crates/console/prometeu-vm/src/object.rs`, `crates/console/prometeu-vm/src/virtual_machine.rs`

### Step 3 - Fix `GET_GLOBAL` without semantic change

**What:** Remove avoidable repeated copy pressure from `GET_GLOBAL`.
**How:** Keep the observable contract identical while changing retrieval/storage internals so already-materialized expensive payloads are not recopied on each hot-path access.
**File(s):** `crates/console/prometeu-vm/src/virtual_machine.rs`

### Step 4 - Optimize string-producing opcode paths

**What:** Reduce repeated allocation/copy pressure in string-producing operations, starting with `ADD`.
**How:** Separate true new-string creation from repeated cloning of existing payloads. Ensure any operation that semantically creates a new string still does so correctly, while avoiding extra transient allocations around existing materialized values.
**File(s):** `crates/console/prometeu-vm/src/virtual_machine.rs`, related helper modules if introduced

### Step 5 - Preserve semantic coverage

**What:** Prove semantics did not change while ownership internals did.
**How:** Add/update tests for constants, dynamic strings, globals, arithmetic/string `ADD`, and edge cases where strings cross stack/global boundaries.
**File(s):** `crates/console/prometeu-vm/src/virtual_machine.rs`, VM test modules

## Criterios de Aceite

- `GET_GLOBAL` keeps its public semantics unchanged.
- Hot-path access to already-materialized expensive values no longer performs avoidable repeated payload copies.
- Numeric hot paths remain allocation-free.
- String operations that truly create a new string remain correct and deterministic.
- VM tests cover constant-pool strings, runtime-created strings, and globals under the new ownership model.

## Tests / Validacao

### Unit Tests

- Value/ownership tests for expensive payload movement or referencing.
- VM opcode tests for `GET_GLOBAL`, `SET_GLOBAL`, and string `ADD`.

### Integration Tests

- Small bytecode programs that read globals repeatedly and mix numeric/string paths.

### Manual Verification

- Review allocation-sensitive hotspots in the final implementation to confirm repeated copies were removed rather than merely relocated.

## Riscos

- Internal representation changes may ripple into GC/root traversal.
- A partial fix may hide copies in helper methods rather than eliminate them.
- Over-optimizing strings could accidentally change guest-visible behavior if semantic boundaries are not defended by tests.

## Dependencies

- `DEC-0018`
- Existing VM heap/object/root traversal contracts